RFC: Turn off ActiveRecord caching within lock blocks #52
Description
I explained this issue in depth in #47. The short version is that ActiveRecord caching can lead to stale query results within an advisory lock, at a time when it's critical to be working with up-to-date data.
I'm thinking that it may make sense for the with_advisory_lock block to automatically turn off caching via ActiveRecord::QueryCache.uncached for the duration of the block. Or perhaps there should be an argument to with_advisory_lock to control this.
The example use case is using an advisory lock to prevent the double-creation of a record. Take this for example:
def create_or_update_user(external_user)
transaction do
with_advisory_lock("create-user-#{external_user.id}", transaction: true) do
user = User.find_or_initialize_by(external_id: external_user.id)
user.update_info(external_user)
user.save!
end
end
endIn this example users are created from 3rd party webhooks, and the goal is to create a new user if it doesn't already exist, or update the existing one.
The problem I ran into is that upstream of the lock in my code I had a User.find_by(external_id: external_user.id). The user didn't exist at this point, so the result was being cached as nil. Then inside the lock find_or_initialize_by used the cached result of nil, even though by this time the user record had already been created, thus defeating the purpose of the lock.
I solve this in my code by using uncached to ensure a fresh query:
def create_or_update_user(external_user)
transaction do
with_advisory_lock("create-user-#{external_user.id}", transaction: true) do
self.class.uncached do
user = User.find_or_initialize_by(external_id: external_user.id)
user.update_info(external_user)
user.save!
end
end
end
end